Drift compensated document sensing system

ABSTRACT

In a document sensing system, the output of the sensor varies in a direct proportion to the area of the sensor device affected by the document, that is, if 50 percent of the sensor area is affected by the presence of the document, the sensor output will be 50 percent of the output obtained when the sensor is completely unaffected by the document. A voltage divider supplies a predetermined fractional amount, e.g. 50 percent, of the sensor output signal to a positive peak follower circuit which determines the peak value of the function of the output of the sensor and charges a capacitor to a reference voltage of proportional value. The sensor output is compared to this reference voltage. A minimum fixed reference voltage is also supplied to the comparator. If the input voltage from the sensor to the comparator rises above the composite reference value, an output signal is provided. If this input voltage is terminated, the output signal will be terminated after a predetermined time, as determined by a delay device.

[ Nov. 12, 1974 DRIFT COMPENSATED DOCUMENT SENSING SYSTEM Leonard J. Wallace, Vestal, NY.

International Business Machines Corporation, Armonk, NY.

22 Filed: Dec. 13, 1973 21 Appl. No.: 424,498

[75] Inventor:

[73] Assignee:

[56] References Cited UNITED STATES PATENTS 3,777,152 l2/l973 Dorsman 250/555 Primary Examiner-Rudolph V. Rolinec Assistant ExaminerB. P. Davis Attorney, Agent, or FirmPaul M. Brannen 571 ABSTRACT In a document sensing system, the output of the sensor varies in a direct proportion to the area of the sensor device affected by the document, that is, if 50 percent of the sensor area is affected by the presence of the document, the sensor output will be 50 percent of the I output obtained when the sensor is completely unaffected by the document. A voltage divider supplies a predetermined fractional amount, eg 50 percent, of the sensor output signal to a positive peak follower circuit which determines the peak value of the function of the output of the sensor and charges a capacitor to a reference voltage of proportional value. The sensor output is compared to this reference voltage. A minimum fixed reference voltage is also supplied to the comparator. if the input voltage from the sensor to the comparator rises above the composite reference value, an output signal is provided. if this input voltage is terminated, the output signal will be terminated after a predetermined time, as determined by a delay device.

5 Claims, 2 Drawing Figures GERABLE o()P MONO- STABLE 4 RMVB PATENTEQ RSV I 2 I974 IP M FIG. 2

RETRIG- GERABLE MONO- STABLE RMVB R4 li DRIFT COMPENSATED DOCUMENT SENSING SYSTEM FIELD OF THE INVENTION This invention relates generally to document sensing systems and has reference in particular to a document sensing system which provides a more accurate indication of the location of the document without problems arising from drift caused by variations in operating voltage, or changes in component parameters.

DESCRIPTION OF THE PRIOR ART SUMMARY OF THE INVENTION Generally stated, it is an object of this invention to provide an improved drift compensated document sensing system.

More specifically, it is an object of this invention to provide a drift compensated document sensing system in which a composite reference voltage corresponding to a given proportional value of the peak output of the sensor and a fixed minimum value is used to determine whether or not an input signal is of sufficient magnitude.

Another object of the invention is to provide a drift compensated document sensing system in which a suitabledelay is interposed in the output of the cricuitry to eliminate the possibility of erroneous operation from tum-on or tum-off spikes in the amplifier circuitry.

Another object of the invention is to provide a drift compensated document sensing system in which a minimum fixed reference voltage is provided in order to prevent the system from being actuated by extraneous noise signals from holes or tears in the documents to be sensed. Y

In practicing the invention in its preferred form, an ultrasonic sensor is provided, which provides an output in the higher sonic frequencies, for example, 40 Khz, when no document is in the presence of the sensor. As the view of the sensor is obscured by the document, the 40 Khz input signal is decreased to zero or some minimum value. This input signal is supplied to a voltage divider circuit which derives a voltage of approximately 50 percent of the value of the input signal. This proportional input is supplied to one input of a high speed comparator circuit, which may be a suitable differential amplifier, the output of which is connected by a first diode back to the second input of the comparator. Also connected to the second input of the comparator is a reference voltage signal fixed in magnitude and determined by the value of circuit components including a second and third diode connected to supply this reference voltage to the second input of the comparator. The second input of the comparator is also connected to a first input of a second high speed comparator circuit, the second input of which is connected'di-rectly to the input signal from the sensor. Thusthe composite reference voltageis compared with the input signal in the second comparator, and the output of the second comparator is supplied via a suitable time delay device, such as a retriggerable monostable multivibrator, to an output circuit which will indicate the presence or absence of the document.

While the invention has been particularly shown and described with reference to a preferred embodiment,

thereof, it will be understood by those skilled in the art that various changes in fonn and details may be made therein without departing from the spirit and scope of the invention.

DESCRIPTION OF THE DRAWING In the drawings, FIG. 1 is a schematic diagram of a document sensing system using ultrasonic waves, of the type involved with the present invention. FIG. 2 is a diagrammatic illustration of the circuitry utilized in the amplifier shown generally in FIG. 1.

Similar reference characters refer to similar parts and each of the views.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIG. 1 of the drawings, there is shown in schematic form a document sensing system of the type herein involved. Alternating current energy at a suitable sonic frequency, for example 40 Khz, is supplied from a conventional generator 3 to an ultrasonic transmitting transducer TT, which may take the form of a conventional piezo electric device. The transducer radiates ultrasonic sound energy at a corresponding frequency of 40 Khz across the intervening space to a receiving transducer RT, where the sonic vibrations are changed to electrical signals, and supplied to the input terminal IT of an amplifier device 5, having an output terminal OP. Normally, the ultrasonic signals from the transmitting transducers travel directly to the receiving transducer, but if a document is moved through the space between the transducers, it will be seen that the ultrasonic sound energy received at the receiving transducer will diminish as the document moves into the space between the transducers and will be reduced to zero or a minimum value as the document obliterates the view of the transmitting transducer by the receiving transducer.

In this type of arrangement, it has been found that the output of the receiving transducer RT as represented by the magnitude of the alternating circuit signal voltage at the terminal IP is directly proportional to the area of the receiving transducer affected by the presence of the document. That is, as the document enters the space between the receiving transducer and the transmitting transducer, the output voltage point I? will be reduced in magnitude in proportion to the area of the receivingtransducer which is obscured by the advancing document so that, for example, when the leading edge of the document has moved to a point halfway across the sensor area of the receiving transducer RT, the output signal voltage will be reduced 50 percent in magnitude.

Referring now to FIG. 2, there is shown a schematic circuit of a preferred embodiment of the invention in which. a composite threshold voltage is derived from the input signal and a fixed minimum voltage. This composite threshold voltage is compared to the input signal, and when-an-output is indicated, the output opcrates a retriggerable monostable multivibrator to supply a signal to the output terminal OF. The input signal at terminal I? is supplied to a first terminal of a differential amplifier comparator '7, by means of a voltage divider comprising the resistors R1 and R2 connected in conventional manner. The resistors R! and R2 are of such value that the output voltage taken from the junction therebetween represents a proportional part of the input voltage at terminal II. An optimum value for this proportional voltage is one-half of the input signal. A bypass capacitor C1 is connected around the voltage divider. The differential comparator 7 is of conventional design and has a ground connection G, and connections to suitable direct current power supplies indicated by the designations +V and V as shown. The output of comparator 7 is supplied via a first diode D1 to a first circuit node CNl, which is also connected to the first input of a second differential comparator 9, as well as being supplied to the second input terminal of comparator 7 as can be clearly seen from the drawings.

The circuit configuration as thus far described operates to rectify the peak value of the proportional part of the input voltage at terminal IP and to supply this rectified output at a value which represents the peak values of the proportional part of the input signal. This rectified peak voltage forms the variable reference voltage used in the subsequent operation of the system.

Superimposed on this variable reference or threshold is a fixed voltage supplied to node CNl from the network comprising diodes D2, D3 and resistor R3. The parts are selected and proportioned so that this minimum reference voltage is such that it will eliminate noise in the threshold circuit occurring at normal values due to noise in adjacent electrical circuitry.

The capacitor C2 is connected between the node CNl and ground and has a value such that it maintains the composite reference voltage for a predetermined time during the operation of the sensor system.

The second input to the differential comparator 9 comprises the input signal at terminal IP as manifest from the drawings. Thus at comparator 9, the input signal is compared with the composite reference voltage comprising the variable threshold voltage superimposed on the fixed threshold voltage, and when the input signal exceeds this threshold voltage value, an output is provided therefrom to the input of a monostable multivibrator l 1, an auxiliary connection being provided to capacitor C3 and therefrom to ground.

Retriggerable monostable multivibrator 11 is of conventional type, provided with a power supply connection designated as +V and utilizing external resistance and capacity elements R4 and C4 to determine the operational timing of the monostable multivibrator.

Upon the supply of an input signal (no document present), the retriggerable monostable multivibrator (RMVB) will be triggered on, and provide an output signal at terminal OP.

With no document present, this output signal will be maintained and is dependent on the input again being triggered before the time out of the timing network R4 and C4. These elements are selected to provide a time period greater than the input signal period.

Upon termination of the input signal, the RMVB output signal at terminal OP will be terminated, after a predetermined time (controlled by timing elements R4 and C4) and is indicative of the sensing of a document between transducers TT and RT.

From the foregoing, it will be apparent that the present invention provides an improved drift free document sensing system by utilizing an arrangement for obtaining a composite threshold for the output signals from the sensor, in which a proportional value of the sensor output signal is amplified to develop a peak voltage variable threshold signal, which is combined with a fixed minimum threshold signal, to thereby determine which of the input signals will be passed by an associated comparator circuit. When the composite threshold or reference voltage is exceeded, a multivibrator causes an output signal to be developed therefrom.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

I claim:

1. A document sensing system comprising, in combination,

a document sensor which produces an electrical sensor output signal proportional to the area of the sensor which is affected by the presence of a document,

a voltage divider connected to said sensor to receive the sensor output signal therefrom and having an output terminal at which a predetermined portion of said sensor output signal appears,

positive peak follower means having an input connected to the output terminal of said voltage divider means and having an output terminal at which appears a voltage proportional to the positive peak values of the predetermined portion of said sensor output signal,

a fixed minimum threshold voltage source,

a voltage storage device,

a first circuit node onncstedt the QQMIQLE of said peak follower means, to said fixed voltage source and to said voltage storage device to thereby provide at said node a composite threshold voltage,

voltage comparison means connected to said first circuit node and to said sensor output to provide a comparison output signal, when said sensor output exceeds said composite threshold voltage; and

delay means having an input terminal connected to output of said comparison means and having an output terminal which provides an output signal which is terminated at a predetermined time following the termination of an input signal.

2. A document sensing system as claimed in claim 1, in which said positive peak follower means comprises a differential amplifier having a first input terminal connected to said voltage divider, and having a second input terminal connected to said first circuit node.

3. A document sensing system as claimed in claim 1, in which said voltage comparison means comprises a differential amplifier.

4. A document sensing system as claimed in claim 1, in which said delay means comprises a retriggerable monostable multivibrator which supplies an output signal following the supply of an input pulse thereto.

5. A document sensing system as claimed in claim 1, in which the output of said peak follower means is connected to said first circuit node by a first diode poled to pass current toward said node, and said fixed threshold source is connected to said first circuit node by a second diode poled to pass current toward said node. 1K It 

1. A document sensing system comprising, in combination, a document sensor which produces an electrical sensor output signal proportional to the area of the sensor which is affected by the presence of a document, a voltage divider connected to said sensor to receive the sensor output signal therefrom and having an output terminal at which a predetermined portion of said sensor output signal appears, positive peak follower means having an input connected to the output terminal of said voltage divider means and having an output terminal at which appears a voltage proportional to the positive peak values of the predetermined portion of said sensor output signal, a fixed minimum threshold voltage source, a voltage storage device, a first circuit node connected to he output terminal of said peak follower means, to said fixed voltage source and to said voltage storage device to thereby provide at said node a composite threshold voltage, voltage comparison means connected to said first circuit node and to said sensor output to provide a comparison output signal, when said sensor output exceeds said composite threshold voltage; and delay means having an input terminal connected to output of said comparison means and having an output terminal which provides an output signal which is terminated at a predetermined time following the termination of an input signal.
 2. A document sensing system as claimed in claim 1, in which said positive peak follower means comprises a differential amplifier having a first input terminal connected to said voltage divider, and having a second input terminal connected to said first circuit node.
 3. A document sensing system as claimed in claim 1, in which said voltage comparison means comprises a differential amplifier.
 4. A document sensing system as claimed in claim 1, in which said delay means comprises a retriggerable monostable multivibrator which supplies an output signal following the supply of an input pulse thereto.
 5. A document sensing system as claimed in claim 1, in which the output of said peak follower means is connected to said first circuit node by a first diode poled to pass current toward said node, and said fixed threshold source is connected to said first circuit node by a second diode poled to pass current toward said node. 